Method and device for detecting abrasions of electrodes in stationary welding gun

ABSTRACT

A method for detecting abrasion of both a moving side electrode ( 2 ) and a fixing side electrode ( 3 ) of a stationary type welding gun in which the moving side electrode ( 2 ) and the fixing side electrode ( 3 ) opposed to the moving side electrode ( 2 ) are provided and a workpiece (W) held by a robot (R) is to be welded, wherein reference positions of electrode ends of both of the electrodes ( 2 ) and ( 3 ) are read by an electrode end detecting device ( 7 ) attached to the robot (R), positions of the electrode ends are then detected by the electrode end detecting device ( 7 ) in a middle of welding, and amounts of abrasion of both of the electrodes ( 2 ) and ( 3 ) are calculated from a difference between the reference positions.

TECHNICAL FIELD

The present invention relates to a method and apparatus for detectingthe abrasion of an electrode of a stationary type welding gun.

BACKGROUND ART

For a stationary type welding gun in which a welding workpiece is heldby the hand of a robot and is fixed to the ground, a moving sideelectrode is moved by a driving source such as an air cylinder or aservomotor (a servomotor for a gun) and a welding object is interposedbetween the moving side electrode and a fixing side electrode making apair, and is pressurized and held and a large current is caused to flow,resulting in the execution of welding. Due to the pressurization andwelding heat in the welding, both of the electrodes making a pair aregradually abraded. For this reason, conventionally, a stationary typegun provided with an equalizing mechanism capable of vertically rockinga welding gun itself by using a spring or a balance cylinder isutilized, or the hand of the robot is caused to comprise a floatingdevice having the same function even if the equalizing mechanism is notprovided, and the former gun carries out the equalization in the weldingor absorption is performed by the hand of the robot as in the lattercase. Thus, a countermeasure has been taken by using a rough weldingmethod capable of ignoring the amount of abrasion of a chip to carry outthe welding using these technical manners without detecting the amountof the abrasion of the chip.

However, there is such a defect that the equalizing mechanism and thefloating device of the hand are expensive. Furthermore, an electricwelding gun using a servomotor has spread. If the amount of abrasion canbe grasped by the robot, it is possible to always maintain thepositional relationship of an electrode chip with respect to a workpieceby taking the amount of abrasion into account to put the workpiece inthe stationary type welding gun. Consequently, it is possible to obtainwelding which is stable in respect of quality of the welding.

DISCLOSURE OF THE INVENTION

Therefore, it is an object of the invention to provide a method andapparatus for detecting the abrasion of an electrode of a stationarytype welding gun which can accurately detect the amount of abrasion ofthe electrode of the stationary type welding gun by means of a robot.

The invention provides a method for detecting abrasion of both a movingside electrode and a fixing side electrode of a stationary type weldinggun in which the moving side electrode and the fixing side electrodeopposed to the moving side electrode are provided and a workpiece heldby a robot is to be welded, wherein reference positions of electrodeends of both of the electrodes are read by an electrode end detectingdevice attached to the robot, positions of the electrode ends are thendetected by the electrode end detecting device in a middle of welding,and amounts of abrasion of both of the electrodes are calculated from adifference between the reference positions.

Moreover, the invention provides an apparatus for detecting abrasion ofan electrode of a stationary type welding gun, wherein an electrode enddetecting device for detecting electrode ends of both a moving sideelectrode and a fixing side electrode of the stationary type welding gunin which the moving side electrode and the fixing side electrode opposedto the moving side electrode are provided is attached to a robot, andthere is provided a calculating section for reading reference positionsof the electrode ends of both of the electrodes by the electrode enddetecting device, then detecting positions of the electrode ends by theelectrode end detecting device in a middle of welding, and calculatingamounts of abrasion of both of the electrodes from a difference betweenthe reference positions.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a view showing the structures of a stationary welding gun anda welding robot according to an embodiment of the invention,

FIG. 2 is a schematic view showing an example of a non-contact sensor asan electrode end detecting device,

FIG. 3 is an explanatory view showing the detection of an electrode endby the non-contact sensor,

FIG. 4 is an explanatory view showing a correcting operation based on anamount of abrasion,

FIG. 5 is a view showing the structures of the stationary type weldinggun and the welding robot according to the invention,

FIG. 6 is an explanatory view showing the detection of the electrode endby a contact and the correcting operation based on the amount ofabrasion, and

FIG. 7 is an explanatory view showing a correcting operation based onthe amount of abrasion in an air type.

BEST MODE FOR CARRYING OUT THE INVENTION

First Embodiment

FIG. 1 is a view showing the structures of a stationary type welding gunand a welding robot according to an embodiment of the invention. Astationary type welding gun 1 comprises a movable side electrode 2 and afixing side electrode 3 opposed to the movable side electrode 2, and themovable side electrode 2 carries out a closing operation and an openingoperation by means of a servomotor 4 to be a driving source. Theoperation of the stationary type welding gun 1 is controlled by awelding controller 5.

A welding robot R comprises a hand 6 for holding a workpiece W, and anon-contact sensor is attached as an electrode end detecting device 7for detecting an electrode end to the terminal of the hand 6 in order todetect the abrasion of the electrodes 2 and 3. The operation of thewelding robot R is controlled by a robot controller 8.

FIG. 2 is a schematic view showing an example of a non-contact sensor tobe the electrode end detecting device. In the non-contact sensor, alight emitting section 7 a for emitting a beam B and a light receivingsection 7 b for receiving the beam B are attached to a forked frame 7 copposite to each other and are coupled to a hand through a bracket 7 d.The elements of the light emitting section 7 a and the light receivingsection 7 b are connected to the robot controller 8 by a sensor cable 7f through an amplifier 7 e for electric conversion. The non-contactsensor detects an electrode end by moving the movable side electrode 2or the fixing side electrode 3 through the robot R in such a directionas to cross the beam B.

FIG. 3 is an explanatory view showing the detection of the electrode endby the non-contact sensor.

The positions of the electrode ends to be the reference positions of themovable side electrode 2 and the fixing side electrode 3 are detectedbefore welding. It is preferable that the positions of the electrodeends to be the reference positions should be set when the electrodes arebrand-new. As shown in FIG. 3 (a), the robot R moves the non-contactsensor 7 to be the electrode end detecting device in such a manner thatthe beam B goes toward the movable side electrode 2. When the electrodeend is detected by the beam B, the robot R carries out search anddetection in a Z+ direction of tool coordinates and the referenceposition of the movable side electrode 3 is read. The reference positionthus read is once stored in the robot controller.

Then, the non-contact sensor 7 is moved in such a manner that the beam Bgoes toward the fixing side electrode 3. The search and detection iscarried out in a Z− direction to read the reference position of thefixing side electrode 3. The reference position thus read is once storedin the robot controller.

When the workpiece W is pressurized and held by both of the electrodes 2and 3 to cause a large current to flow and to repeat welding,thereafter, the electrodes 2 and 3 are gradually abraded. For thisreason, it is necessary to detect the amounts of abrasion of the movableside electrode 2 and the fixing side electrode 3 during the operation ofthe robot R and to correct the amounts of movement of the movable sideelectrode 2 and the robot R. When the search and detection is carriedout in the same manner as the detection of the reference position forthe movable side electrode 2 and the fixing side electrode 3 after thewelding, the position of the abraded electrode end which is detected isdetected. An amount LU of abrasion of the movable side electrode and anamount LW of abrasion of the fixing side electrode are obtained from adifference between the position of the abraded electrode end thusdetected and the reference position.

Next description will be given to the relationship between the amountsLU and LW of abrasion thus detected and the operations of the movableside electrode and the robot.

FIG. 4 is an explanatory view showing a correcting operation based onthe amount of abrasion.

In case of an electric type welding gun shown in FIG. 4, the amount LUof abrasion of the movable side electrode and the amount LW of abrasionof the fixing side are converted to the amount of shift of the motor ofthe stationary type welding gun in order to adapt the workpiece W heldby the hand 6 of the robot R to an abrasion correcting direction in thewelding attitude of the stationary type gun, and the movable sideelectrode is shifted and moved in such a direction as to pressurize theworkpiece W. The direction is a closing direction by the total amount ofLW and LU in a manual manipulation or an automatic operation.

Moreover, the amount LW of abrasion of the fixing side is converted tothe amount of shift of the tool coordinates of the hand of the robot,and the workpiece W held by the hand 6 is moved by LW in the Z−direction of the tool coordinates through the robot R.

All position data on both of the electrodes 2 and 3 obtained by thedetecting operation and a position data difference are position data onthe robot R. When they are transmitted to the component of the amount ofmovement of the movable side electrode, it is necessary to convert therobot position data into movable side electrode position data becausethe resolution of the movable side electrode, i.e., a distance by whichthe movable side electrode proceeds with respect to the number ofrotations of the motor is defined.

Second Embodiment

FIG. 5 is a view showing the structures of the stationary type weldinggun and the welding robot according to the invention, and a stationarytype welding gun 1 comprises a movable side electrode 2 for carrying outclosing and opening operations by means of a servomotor 4 to be adriving source, and a fixing side electrode 3 provided opposite to themovable side electrode 2.

A welding robot R comprises a hand 6 for holding a workpiece W, and acontact 9 to be an electrode end detecting device for detecting anelectrode end is attached to the terminal of the hand 6.

As shown in FIGS. 6(a) and (b), when the contact 9 comes in contact withthe electrode end, i.e., the contact 9 and the electrode 2 or 3 areelectrically conducted in the embodiment, the position coordinates ofthe robot in that position are stored in a robot controller.

In the calculation of an amount of abrasion, an amount LU of abrasion ofthe movable side electrode 2 and an amount LW of abrasion of the fixingside electrode 3 are obtained from a difference between the coordinatesof the reference positions of the movable side electrode 2 and thefixing side electrode 3, i.e., the reference position of the robotobtained when the contact comes in contact with each of the electrodeends and the coordinates of the position of the robot obtained when thecontact comes in contact with the tip of an electrode chip afterwelding.

As shown in FIG. 6(c)), based on the amounts LU and LW of abrasion thuscalculated, the movable side electrode 2 is shifted by LU+LW and thehand is shifted by LW toward the fixing side electrode 3 side.

While the application to the stationary type gun using the servomotor 4as a driving source has been described above, brief description will begiven to detection to be carried out in case of a conventional type aircylinder or a stationary type gun using an oil pressure with referenceto FIG. 7.

In a detecting operation, the detection is carried out by an electrodeend detecting device in which a movable side electrode and a fixing sideelectrode are attached to a hand in the same manner as in the first orsecond embodiment. When a correcting operation is to be carried out asshown in FIG. 7, it is sufficient that the robot R is simply caused tocorrect the workpiece W by the amount LW of abrasion on a fixing side ina tool coordinate—direction differently from an electric operation type.

By detecting the amount of abrasion of the electrode of the stationarytype gun through the method described above and carrying out shift,i.e., correction in welding and pressurization by the amount ofabrasion, quality of the welding can be stabilized still more.

Moreover, the amounts of abrasion of both electrodes are monitored by acontroller. When the threshold of the abrasion is reached, therefore,the exchange of the electrode can also be demanded on the spot. In thecase in which the amount of abrasion of a chip is rapidly increasedexcessively in the detection, furthermore, it is a matter of course thatchip missing can technically be detected.

In addition, the electrode end detecting device can also be attached tothe complicated hand of the robot without great cares and amaintainability can also be prevented from being hindered. In respect ofa cost, there is such an advantage that the cost can be considerablyreduced, for example, the gun can have an equalizing function or thehand can have a floating device.

While an electric type welding gun using a servomotor has spread, it ispossible to carry out pressurization in a state in which the positionalrelationship of an electrode chip for a workpiece can always bemaintained by grasping the amounts of abrasion of the fixing sideelectrode and the movable side electrode with the hand of a robot havinga detecting device which is easily attached to the robot and taking theamount of abrasion into account to put a workpiece in a stationary typewelding gun and to carry out correction. Consequently, it is possible toobtain welding which is stable in respect of quality of the welding.Although the correction is carried out over only the fixing sideelectrode in air type and hydraulic stationary guns, moreover, thequality of the welding can be more enhanced than that in conventionalnon-correcting pressurization.

In the invention, furthermore, expensive means such as an equalizingmechanism or a floating device is not required but there is such anadvantage that teaching and detection can easily be carried out by arobot and an electrode can readily be managed.

INDUSTRIAL APPLICATION

The invention is useful for a method and apparatus for detecting theabrasion of an electrode of a stationary type welding gun.

1. A method for detecting abrasion of both a moving side electrode and afixing side electrode of a stationary type welding gun, wherein thefixing side electrode is provided at a position opposed to the movingside electrode, and a workpiece held by a robot is to be welded, themethod comprising: reading reference positions of electrode ends of bothof the electrodes, by an electrode end detecting device attached to therobot, prior to welding, detecting positions of the electrode ends, bythe electrode end detecting device, during welding, and calculatingamounts of abrasion of both of the electrodes from a difference betweenthe reference positions and the detected positions.
 2. An apparatus fordetecting abrasion of an electrode of a stationary type welding gun,comprising: an electrode end detecting device for detecting electrodeends of both a moving side electrode and a fixing side electrode of thestationary type welding gun, wherein the fixing side electrode isprovided at a position opposed to the moving side electrode, and theelectrode end detecting device is attached to a robot, and a calculatingsection for reading reference positions of the electrode ends of both ofthe electrodes by the electrode end detecting device, then detectingpositions of the electrode ends by the electrode end detecting deviceduring welding, and calculating amounts of abrasion of both of theelectrodes from a difference between the reference positions and thedetected positions.